This invention relates to a pressure-applied type semiconductor device and, more particularly, to a structure of a pressure-applied type semiconductor device having improved reliability.
Pressure-applied type semiconductor devices, in which semiconductor elements such as transistors, thyristors and gate turn-off thyristors (to be referred to as GTOs hereinafter) are pressed to form an electrical connection, have been well known in the art and generally used for power supplying purposes. FIG. 1 shows a typical construction of a pressure-applied type semiconductor device of this sort. On opposite sides of a semiconductor body 10, cylindrical metal stamps 16 and 18 having high thermal and electrical conductivities are provided via metal plates 12 and 14 having a coefficient of thermal expansion close to that of the body 10. The semiconductor body 10 is pressed by the metal stamps 16 and 18 in the directions indicated by the arrows. The metal plates 12 and 14 are provided between the semiconductor body 10 and the metal stamps 16 and 18 in order to prevent mechanical stress from being applied to the semiconductor body 10 due to the bimetal effect between the semiconductor body 10 and the metal stamps 16 and 18. The bimetal effect results from the difference in the coefficient of thermal expansion between the body and metal stamps. Where the semiconductor body 10 consists of silicon (Si), the metal plates 12 and 14 are usually made of molybdenum (Mo) or tungsten (W). These plates may be called thermal compensation plates. Where the semiconductor body 10 comprises a silicon thyristor, the anode electrode side metal plate 14 is made of a molybdenum plate and is directly bonded to the element by the alloying method. For the metal stamp 18, a cylindrical copper stamp is used and is secured to the metal plate 14 via a solder layer 20. On the cathode electrode side, the metal stamp 16 is likewise secured to the metal plate 12 of molybdenum or the like via a solder layer 22, or a braze layer.
A thyristor or diode that is formed as a power semiconductor body has its cathode electrically integrated as shown in FIGS. 2A and 2B. Thus, even if the press contact is slightly lacking in uniformity, the same performance or behavior could be obtained as when the electric contact is uniform. Lack of uniformity of the press contact, therefore, has presented no substantial problems. FIG. 2B is a sectional view taken along line A-A' in FIG. 2A. In FIGS. 2A and 2B, the element designated by reference numeral 24 is the semiconductor body, 26 is the cathode electrode, 28 the gate electrode, and 30 the anode electrode.
On the other hand, in high power semiconductor devices including transistors or GTOs that have been recently developed, the cathode is divided into a plurality of sections as shown in FIGS. 3A and 3B. These divided electrodes 32 on the cathode region are pressed by a copper stamp via a molybdenum or tungsten plate 34 which is provided inside a dashed circle as shown in FIG. 3A. In the case of a GTO, each set of the semiconductor layers that are formed under the individual cathode electrodes 32 is operated in parallel as a GTO to control a large current. The entire cathode electrode is thus uniformly pressed to prevent unbalanced currents among cathode elements due to the local press contact or contact resistance differences and also prevent unbalanced currents due to fluctuations of the turn-off characteristics resulting from lack of uniformity of the stress in the silicon. FIG. 3B is a sectional view taken along line B-B' in FIG. 3A.
In the pressure-applied type semiconductor devices described, the stress distribution over the semiconductor body is basically equivalent to that in a case where a semi-infinite plate is pressed by a rigid body. On the basis of this assumption, experiments were conducted, in which a semi-infinite elastic member 36 was pressed with a cylindrical rigid post 38 as shown in FIG. 4A. The stress P(Z) provided in the semi-infinite elastic body 36 in the direction normal to the press contact surface is analyzed according to a program which is prepared for the purpose of analyzing stress in complex structures, commonly called SAP (Structural Analysis Program). The result is as shown in FIG. 4B. (The SAP was developed in 1969 by Prof. E. L. Wilson and others at the University of Southern California, and rearranged forms of this prototype program have been used in various areas in the world.) As is apparent from FIG. 4B, the stress distribution over the surface of contact between the semi-infinite elastic body 36 and rigid body 38 is such that the stress is theoretically infinite at the edge of the rigid body so that the stress distribution in the semi-infinite elastic body 36 is extremely lacking in uniformity.
From this fact, it is thought that if a semiconductor body (i.e., a semi-infinite disc) is pressed with a metal plate and a copper stamp, the stress distribution in the semiconductor body is as shown in FIG. 4B. In general, with the prior art pressure-applied type semiconductor device, the stress concentration at the edge of the semiconductor body is inevitable. In fact, when a GTO device which was ruptured during use is disassembled, a ring-like trace of press contact as shown by the dashed circle in FIG. 3A can be observed. In the GTO device where such a trace of press contact is observed, the maximum anode current (i.e., maximum controllable anode current I.sub.TGQ) during use is greatly reduced. From the experimental fact that lack of uniformity of the stress distribution causes lack of uniformity of anode current in the press contact surface, resulting in fluctuations of the turn-off time, it may be concluded that the extreme reduction of the maximum anode current results from an overcurrent density state of the edge where the turn-off time is extended due to high pressure. Further, the GTO device where a trace of press contact can be observed is prone to fatal characteristic deteriorations such as a short-circuit between the cathode and gate electrodes along the edge of the cathode electrode caused by thermal fatigue during operation. However, there has been no appropriate method of solving the above problems.
It has been a common practice to grind a metal plate 42 in contact with a semiconductor body 40 to form an inclined surface 44 as shown in FIG. 5. This is done for the purpose of removing burrs from the metal plate 42 on the side thereof in contact with the body. It seems that the inclined surface can alleviate the stress at the edge. However, the grinding angle .theta. is usually .theta..gtoreq.30.degree., and the thickness of the ground portion is usually 100 to 300 .mu.m with a metal plate 42 having a thickness of 500 to 1,000 .mu.m. The height of the cathode region in the semiconductor body, on the other hand, is 10 to 30 .mu.m. Therefore, the length of the ground portion is far greater than the height of the cathode region. This means that the removal of burrs reduces a post diameter, that is, changes the outermost point of the contact surface from point P to point Q as shown in FIG. 5.
In this case, the same result of analysis of stress distribution as that shown in FIG. 4B could be obtained using the SAP as mentioned above. In this case, for the same load applied, the stress at the edge is found to be increased by an amount corresponding to the reduction of the area of the metal plate in contact with the semiconductor body 40. That is, the formation of the inclined surface 44 makes various conditions for the semiconductor body inferior.
As has been shown, with the prior art pressure-applied type semiconductor device, the stress distribution in the semiconductor body always lacks uniformity, giving rise to non-uniform electrical characteristics. These non-uniform electrical characteristics sometimes lead to the rupture of the semiconductor body.